In agricultural machines such as, for example, telescopic loaders, wheel loaders or front loaders on tractors, it is common practice to apply a hydraulic spring support system that provides spring support for the boom or the oscillating crane in order to attain an improved total spring support comfort on the vehicle, particularly during operation. Here the lifting side of a hydraulic cylinder is connected to a hydraulic accumulator in order to attain a spring support through the hydraulic accumulator. Moreover the lowering side of the hydraulic cylinder is connected to a hydraulic tank in order, on the one hand, to avoid cavitation on the lowering side of the hydraulic cylinder and on the other hand to permit a free movement of the piston rod during the spring support process. To increase the safety against a sudden sinking of the boom or the oscillating crane these spring support systems are equipped with load holding valves or pipe break safety arrangements to insure the hydraulic cylinder against breaks in the hoses. However, then it is necessary for the lowering of the hydraulic cylinder to close the tank connection on the lowering side of the hydraulic cylinder, so that a necessary pressure can be built up to open the load holding valve. Only when the load holding valve is opened, can hydraulic fluid drain out of the lifting side of the hydraulic cylinder.
In EP 1157963 A2 a spring support system for the boom of a telescopic loader is disclosed that provides a load holding valve or a pipe break safety arrangement to secure the boom against a lowering. In order to effect, on the one hand, a pressure controlled lowering of the boom that requires an opening of the load holding valve and, on the other hand, to provide a spring support function even in the neutral position of the hydraulic cylinder, a separate selector valve is arranged. A pressure relief valve is provided for the opening of the load holding valve that can be controlled via a control pressure line of the supply line of the lowering side. In order to open the load holding valve the separate selector valve must then be closed, so that the supply line to the tank is closed and the pressure needed to open the load holding valve can build up in the supply line. The disadvantage here is that the pressure necessary to open the relief valve requires relatively high hydraulic power that must be provided each time the hydraulic cylinder is lowered under pressure. Moreover when the spring support is activated precise positioning during the lowering of the boom is made more difficult since the opening pressure on the lowering side of the hydraulic cylinder also indirectly leads to a hydraulic loading of the hydraulic accumulator which is subsequently unloaded, and is, in turn connected with a movement of the hydraulic cylinder.
Accordingly there is a clear need in the art to improve a spring support system of the aforementioned type in such a way that a lowering of the hydraulic cylinder under pressure can be performed with lower hydraulic power and a more precise positioning of the boom becomes possible when this spring support system is activated.